1. Related Applications
This application is a continuation in part of U.S. Pat. application Ser. No. 162,053, filed Feb. 29, 1988, entitled "Improved Method for Measuring The Size And Velocity of Spherical Particles Using The Phase And Intensity of Scattered Light", now abandoned.
2. Field of the Invention
The present invention relates to particle size and velocity measurements using scattered laser light detection and, more specifically, relates to such measurements utilizing the Doppler difference frequency, relative signal phase, and intensity of the scattered light.
3. Art Background
There is a need for the detailed measurement of the size and velocity of spherical particles, drops, bubbles, and the like. Areas of application for such measurements include spray nozzle manufacturing, spray combustion research, application of agricultural pesticides and irrigation, aircraft icing studies, atmospheric aerosol research, planetary studies, fuel analysis, and numerous other applications. Several techniques employing laser light scattering have been considered and developed to determine the size and velocity of particles, drops, bubbles, or the like. These techniques include using the intensity of scattered light by particles, particle visibility and the phase/doppler technique for measuring particle size. Each method has had varying degrees of success when applied in real world environments.
Particle size is determinable from the intensity of the light scattered by particles. The higher the intensity of scattered light, the larger the particle size. In one intensity measurement method, the particle size is computed by assuming that a particle scatters light in proportion to the diameter of the particle squared (d.sup.2). A more precise method is the well known Lorenz-Mie theory. Using the Lorenz-Mie theory, the light scattering intensity can be predicted for uniformly illuminated spherical particles of arbitrary size. For further information on particle measurements using the intensity technique, see van de Hulst, Light Scattering By Small Particles (Dover Publications, 1957). However, particle size measurements which use the intensity of scattered light to determine particle size are quite imprecise because there are a number of unknown parameters such as the incident intensity on the particle, the crosssection of the incident laser light and the particle trajectory through the laser beam. Another method based on light scattering interferometry, referred to as visibility, has been used to measure spherical particles, drops, bubbles, or the like. This method is described by William D. Bachalo, in an article entitled, "Method for Measuring the Size and Velocity of Spheres by Dual-Beam Light-Scatter Interferometry", Applied Optics, Vol. 19, Feb. 1, 1980 and in U.S. Pat. No. 4,329,054 which issued on May 11, 1982. The spatial period of the interference fringe pattern generated by a spherical particle, drop, bubble, or the like as it passes through a sample volume defined by the intersection of crossed laser beams is used in determining the particle size and velocity. Several methods have been devised for measuring the spatial period of the fringe pattern. In the above cited references, the fringe pattern was integrated over the receiver lens aperture to obtain the spacing or spatial period of the fringe pattern. The signal visibility which resulted could then be related to the particle size. This method has drawbacks since the dynamic range of the system was limited, and the combined light scattering by the mechanisms of refraction and reflection produced uncertainties in the measurements. Furthermore, other particles passing through the crossed beams produce extinction pulses that tend to distort the signals and hence, compromise the measurement accuracy.
An alternative approach to the visibility method, referred to as the "phase/doppler method", was described by F. Durst and M. Zare in a paper entitled, "Laser Doppler Measurements in Two-Phase Flows", Proceedings of the LDA Symposium, Copenhagen, 1975. The authors provided a basic analysis using a simple geometrical approach to show that the shape and spacing of the fringes formed by the scattered light through reflection and refraction are functions of the angle between the incident laser beams, their wavelength, as well as the direction of light collection and particle diameter. Although the authors claimed that spherical particles could be measured using a double photo-detector apparatus, they later recognized that size measurements required that the distance between the photo-detectors be matched to the expected fringe spacing produced by the scattered light. They concluded that the method was not practical for particle field measurements.
More recently, the method was discussed by, W. D. Bachalo and M. J. Houser in an article entitled "Phase/Doppler Spray Analyzer for Simultaneous Measurements of Drop Size and Velocity Distributions", Optical Engineering, Vol. 23, No. 5, 1984. In this article, a more rigorous description of the light scattering theory described by W. D. Bachalo in an earlier article entitled, "Method for Measuring the Size and Velocity of Spheres by Dual-Beam Light Scatter Interferometry", Applied Optics, Vol. 19, 1980, was used in the analysis. The theoretical description and experimental verification showed that the method of using signal phase measurements could be used for practical particle field measurements. This was made possible with the selection of appropriate detector separations, on-line observation of the measurements, the use of pairs of detectors, and a single lens system for scattered light detection. The technique was disclosed in U.S. Pat. No. 4,540,283. A similar method was disclosed in U.S. Pat. No. 4,701,051. However, the latter disclosure describes a system using three or more separate receiver lenses and detector systems. The approach disclosed in U.S. Pat. No. 4,701,051 has proved very difficult to operate since each receiver must be carefully aligned to the same measurement point.
Both approaches suffer from the effects of combined light scattering due to reflection and refraction by the particle. This problem was addressed by W. D. Bachalo and M. J. Houser in their report entitled, "Analysis and Testing of a New Method for Drop Size Measurement Using Laser Light Scatter Interferometry", NASA Contract Report No. 174636. The problem was later addressed by Saffman in a report entitled, "The Use of Polarized Light for Optical Particle Sizing", presented at the Third International Symposium on Applications of Laser Anemometry to Fluid Mechanics held in Lisbon, Portugal on July 7-9, 1986. Saffman suggested that a light scatter detection angle of approximately 70.degree. was necessary to avoid errors due to mixed component light scatter detection. This method has the disadvantage of relatively low scattering intensity, lower sensitivity to particle size and inconvenience in applications requiring traversing the sample volume with restricted optical access. Often, backscatter light detection is desirable. Although off-axis backscatter detection has been demonstrated as a viable configuration, errors can occur as a result of the multiple component scattering of reflection and refraction.
The problem is exacerbated when using highly focused laser beams having Gaussian beam intensity distributions. Highly focused beams are required to reduce the sample volume size when coping with high particle number densities. For example, at a light detection angle of 30.degree. with the appropriate polarization, the scattered coefficient for refraction is approximately 80 times that for reflection. However, with a focused beam diameter similar to the sphere diameter and on certain trajectories, the relative incident intensities can be such that the light scattering by reflection and refraction are nearly equal. Because the sign of the phase shift for the fringe pattern produced by reflected light is opposite to that produced by refracted light, the fringes produced by reflection move in the opposite direction.
The present invention discloses a method to overcome this source of error and to provide an alternative means to test the measurements for their accuracy. In addition, the method can provide an alternate means to allow the measurements over several fringes (N.times.2.pi.) without ambiguity, and without using additional phase measurements which can complicate the signal processing. A method is also described for measuring the sample volume cross section which is known to vary with particle size.